The velocity dispersion of stars in the solar neighbourhood thin discincreases with time after star formation. Nordstrom et al. (2004) is the mostrecent observational attempt to constrain the age-velocity dispersion relation.They fitted the age-velocity dispersion relations of each Galactic cardinaldirection space velocity component, U (towards the Galactic centre), V (in thedirection of Galactic rotation) and W (towards the North Galactic Pole), withpower laws and interpreted these as evidence for continuous heating of the discin all directions throughout its lifetime. We re-visit these relations withtheir data and use Famaey et al. (2005) to show that structure in the localvelocity distribution function distorts the in-plane (U and V) velocitydistributions away from Gaussian so that a dispersion is not an adequateparametrization of their functions. The age-sigma(W) relation can however beconstrained because the sample is well phase-mixed vertically. We do not findany local signature of the stellar warp in the Galactic disc. Vertical discheating does not saturate at an early stage. Our new result is that a power lawis not required by the data: disc heating models that saturate after ~ 4.5 Gyrare equally consistent with observations.
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